1. Field of the Invention
The invention relates to a driver for driving the pixels of a display panel. The invention also relates to a display module comprising such a driver, an apparatus comprising such a display module and a method of providing an LCD overdrive drive scheme.
2. Description of the Related Art
LCD display modules are increasingly used for displaying motion pictures and TV signals. Fast moving objects within a picture are a challenge to an LCD display module.
The reason is the response time of the pixels of an LCD display module to a required change in luminance. An overdrive technique is known to improve the response time.
Without overdrive, when a luminance change of a pixel is required, a drive voltage is applied to the pixel such that a desired luminance will be reached in the end. The luminance of the pixel gradually changes from a starting luminance to the desired luminance. If motion pictures or TV signals have to be displayed, the required change in luminance needs to be achieved within a short time period, the so-called frame period. The frame period is the duration during which a single image of a motion picture or TV signal is supplied to the display module. During the frame period, all the pixels of the display panel are addressed once to receive a drive voltage.
When applying the drive voltage, necessary for achieving the desired luminance to the pixel, the actual luminance of the pixel lags behind the desired luminance, due to the inertia of the pixel. It may take several addressing periods until the desired luminance is achieved, causing blurred edges or ghost images.
To shorten the response time of a pixel, an overdrive voltage is applied. The level of the overdrive voltage exceeds the level of the drive voltage required to obtain the desired luminance in the end, and thus targets an overdrive luminance exceeding the desired luminance. When applying the overdrive voltage it usually takes several addressing periods until the overdrive luminance would be achieved. However, when the overdrive voltage is selected carefully, the luminance achieved at the end of a single addressing period is equal to the desired luminance.
Overdrive is recognized as a fundamental requirement for an AMLCD when good motion fidelity is expected. Single-frame LC response at 60 Hz will not be sufficient in future because a number of driving schemes aimed at removing sample-and-hold motion artefacts rely on higher frame rates of 120 Hz or more, thus increasing the need for overdrive.
With an overdrive technique, the desired luminance is reached within one addressing period and thus the response time of the pixel is artificially increased. The overdrive voltage required to achieve the desired luminance depends on the required luminance change and the starting luminance, and further depends on other variables, for example, on the type of display module and the frame rate at which the display is operated. Therefore, the overdrive voltages usually are listed in Look Up Tables (LUTs).
Typically, a unique look-up table is needed for every AMLCD design and possibly adjustments are required batch to batch or even module to module. Furthermore the LUT must vary with ambient temperature and display frame rate if overdrive accuracy is to be maintained. At present, it is by no means clear how much overdrive inaccuracy is tolerable in many applications and therefore how much LUT data must be stored if a framerate-flexible and/or temperature-compensated system is to be implemented.
The standard approach to implementing overdrive is to measure LUT in the factory for each module design (batch, module) and store these in (EP)ROM in the AMLCD module or elsewhere in the system. This creates significant logistical challenges for the manufacturer and also forces a performance compromise because there is a need to trade off overdrive accuracy against the cost of ROM, temperature sensors etc. Thus, implementing overdrive as an integral part of an AMLCD module is a difficult logistical challenge for the module maker due to the specialist measurements that need to be performed for reach new module design, possibly each new batch coming off the production line or even individually for each module. This is in addition to the challenge of storing enough measurement data to ensure sufficiently accurate overdrive for the application. The latter is more important for portable devices where the intended operating temperature range is likely to result in the requirement for temperature compensated overdrive.